Self-stratified aqueous biphasic Zn–I and Zn–Br batteries enabled by spontaneous phase separation and halogen extraction effects of ionic liquids

Self-stratified liquid electrode batteries are considered as a viable solution for large-scale energy storage applications due to their high safety and low cost. However, achieving long-term operation stability with high efficiency of selective ion migration/separation in multiple liquid phases remains a challenge. Here, we report an aqueous biphasic system based on imidazolium ionic liquids (ILs) for constructing membrane-free self-stratified aqueous biphasic Zn–I and Zn–Br batteries. The limited intersolubility and density difference between the IL-dominated catholyte and the aqueous anolyte create a self-stratified liquid-liquid phase separation structure, promising a reliable battery construction. Molecular dynamics simulations identify the internal association of IL, which forms a 3D H–bond network with the C–H···O interaction being the primary contributor, along with a thermodynamic electron transfer of ≤0.03 e. Additionally, a unique π+–bond effect is found between the positively charged imidazole ring of cation and the anion of IL. The as-constructed self-stratified aqueous biphasic Zn–halogen batteries work stably without the necessity of air isolation and exhibit long-term cycling stability (with a capacity mentation of 81.0 % for Zn–I batteries and 76.8 % for Zn–Br batteries over 1000 cycles), high energy efficiencies (93.5 % for Zn–I batteries and 86.7 % for Zn–Br batteries), inherent fire resistance and good scalability, demonstrating great potential for large-scale energy storage applications.